This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-119468 filed Apr. 20, 2000.
1. Field of the Invention
The present invention relates to an electric load driving device for supplying a drive current to an electric load in response to a control signal supplied from a microcomputer and, more particularly, to an electric load driving device for switching output mode of the drive current to the electric load between a high-side output mode and a low-side output mode.
2. Related Art
A vehicular electronic control unit, for instance an engine control unit or a transmission control unit mounted on a vehicle, controls an object by driving a variety of electric loads such as relays or solenoids in accordance with the running state of the vehicle. However, the drive mode for supplying drive currents to the electric loads is different according to the kinds or applications of the electric loads and adopts either a high-side drive mode or a low-side drive mode.
In the case of the high-side drive mode, more specifically, the electric load is connected at its one end with the low potential side (i.e., the ground potential or the potential of the negative terminal of a storage battery generally in the vehicle) of a load power source and at its other end with the electronic control unit, so that the drive current flows out to the electric load from the side of the electronic control unit. In the case of the low-side drive mode, on the other hand, the electric load is connected at its one end with the high potential side (i.e., the battery voltage or the potential of the positive terminal of the battery generally in the vehicle) of a load power source and at its other end with the electronic control unit, so that the drive current flows in from the electric load to the side of the electronic control unit.
In U.S. Pat. No. 5,828,247, for example, there is proposed an electric load driving integrated circuit (IC) which is enabled by a program to switch the output mode of a drive current to the electric load between a high-side output mode and a low-side output mode. This electric load driving device is provided with one N-channel MOSFET as an output transistor for supplying the drive current to the electric load.
Here, the above electric load driving device is provided with a volatile register for storing output mode setting data indicating the output mode to be adopted by the IC. Each time of the power-on for starting the feed of the operating power, the output mode setting data are inputted from the outside and stored in the register so that the output mode may be set to either the high-side output mode or the low-side output mode in accordance with the data stored in the register.
Where the above electric load driving device is mounted together with a microcomputer on an electronic control unit thereby to control the energization of the electric load in accordance with a control signal applied from the microcomputer, therefore, the microcomputer outputs the output mode setting data to the electric load driving device at each power-on thereby to fix the output mode of the electric load driving device.
In this case, however, no problem arises for the time period while the microcomputer normally operates. Where the electric load to be energized is one to be used for controlling the engine, such as the igniter for igniting air-fuel mixture in the engine, however, there arises the following problem.
First of all, generally, the electric load to be used for controlling the engine of the vehicle has to be subjected to a backup control. In this control, the electric load is energized according to a backup control signal outputted from a backup control circuit different from the microcomputer, where the microcomputer does not normally operate, that is, when a stable operating voltage may not be supplied to the microcomputer at the engine starting time as the battery voltage falls or when the microcomputer erroneously executes programs.
In this electric load driving device, the output mode cannot be set to a proper one of the high-side output mode and the low-side output mode, unless the microcomputer operates normally, so that the backup control cannot be made.
The present invention therefore has an object to provide an electric load driving device capable of reliably executing a backup control of an electric load, even when an external circuit such as a microcomputer does not normally operate although the output mode of a drive current to the electric load can be arbitrarily set to either a high-side or low side output mode.
In a specific description, the electric load driving device may be employed in the following manners (1) to (3).
(1) A control signal switching logic circuit is provided outside the IC.
Specifically, this logic circuit receives from the outside: two control signals A and B of a first one A and a second one B instructing the energization and deenergization of an electric load; and a switch signal C indicating which one of the two control signals A and B is to be actually employed for the energization control of the electric load, and supplies the first control signal A as a practically employed control signal to input means of the IC, where the switch signal C is at the logic level indicating the employment of the first control signal A, and the second control signal B as a practically employed control signal to the input means of the IC, where the switch signal C is at the logic level indicating the employment of the second control signal B.
(2) Moreover, the electronic control unit, on which the IC is mounted, is provided with a backup control circuit.
Specifically, this backup control circuit checks whether or not the microcomputer normally operates, and outputs the switch signal at the logic level indicating the employment of the first control signal A, where it is determined that the microcomputer normally operates, the switch signal C at the logic level indicating the employment of the second control signal B, where it is determined that the microcomputer does not normally operate, and a backup control signal instructing the energization and deenergization of the electric load in place of the microcomputer.
(3) The control signal outputted from the microcomputer is inputted as the first control signal A to the logic circuit of (1); the backup control signal outputted from the backup control circuit of (2) is inputted as the second control signal B to the logic circuit of (1); and the switch signal C outputted from the backup control circuit of (2) is inputted to the logic circuit of (1).
Thus, where the microcomputer normally operates, its control signal is supplied to the input means of the IC so that the electric load is supplied with the drive current by the IC in accordance with the control signal supplied from the microcomputer. Where the microcomputer does not normally operate, on the other hand, the backup control signal outputted from the backup control circuit of (2) is supplied to the input means of the IC so that the electric load is supplied with the drive current by the IC in accordance with the backup control signal supplied from the backup control circuit. Therefore, it is possible to execute the energization control of the electric load by the microcomputer and the backup control of the electric load of the case in which the microcomputer does not operate normally.